[Lewis Thomson (OP)]

Pete needs someone to explain why this situation is the case.

"What's the point on smacking into an asteroid when variations in density, hardness, strength and topography could affect the result?"

Can you help them? Leave your answers in the comments below...

[Janus]

Is this in regards to the DART mission?  DART is a 'test of concept' mission to see how feasible the "deflection by collision" method is. 
As far as the different variables go, sure they'd effect exactly how precisely you'd deflect the asteroid, But when it comes to deflecting a Earth-impact asteroid, precision isn't really the issue.  It wouldn't matter if it missed the Earth by 1,000 km or 10,000 km, as long as it missed.  So any real attempt to deflect an asteroid would want to err on the generous side, so that these variables aren't an significant factor.

[evan_au]

Part of the goal is to get a small sample (small n) of the structure of asteroids/comets, to inform future missions that might need to deflect an object on a collision course with Earth.

This has been done before - several missions have smacked into various comets or asteroids with the intent of sampling the surface material. These have:
- Passed through the tail of a comet and collected dustgrains eg Stardust
- Landed on the surface and returned samples, eg Hayabusa2
- crashed at high speed to create a crater and peer beneath the surface eg DCAM-3
- On the Philae mission to comet 67P/Churyumov–Gerasimenko, the surface was so soft and fluffy that the lander failed to detect that it had actually touched down, and bounced along the surface, eventually landing in a ditch.

The most symbolic of these must be NASA's Deep Impact, which struck comet 9P/Tempel at high speed on 4th July 2005.

https://en.wikipedia.org/wiki/List_of_Solar_System_probes#Comet_probes
https://en.wikipedia.org/wiki/List_of_Solar_System_probes#Asteroid_probes

It's cheaper than sending Bruce Willis on a one-way trip to the outer solar system

[alancalverd]

The density etc of the missile (not a satellite, which by definition is an orbiting body)  is irrelevant. The object of the deflection exercise is to transfer momentum to the target, and that would be the same if the missile was made of cheese or uranium, as long as the mass and velocity were the same.

[Eternal Student]

Hi.

"What's the point on smacking into an asteroid when variations in density, hardness, strength and topography could affect the result?"

   
    The point is clear and simple:  We have to try,  we have to get data and we have to find out what factors might be important. 

   Quote from a recent Nakedscientists podcast:    https://www.thenakedscientists.com/podcasts/short/do-asteroids-pose-real-threat
  Hugh James summarises the situation about collisions with earth:     "Let me put it this way, it's not a case of   IF  but  WHEN  .."

Best Wishes.